The present invention relates in general to control valves and in particular to a control valve for keeping constant viscosity and temperature of a pressure medium in a hydraulic circuit by means of throttling the medium on a control slider and by controlling the throttling effect by means of a temperature sensitive control element which is arranged in a preliminary control valve including additional throttling passage controlled by the control element.
The control valve of this type is known for example from the copending patent application corresponding to the German patent application No. P 29 32 481.0. In this known control valve the temperature sensitive control element including a substance expanding or contracting in response to temperature changes controls a throttling passage in the preliminary control valve which in turn regulates the flow in the valve body. In the event of a malfunction of the temperature sensitive control element for example due to an accidental escape of the temperature feeling substance, the control valve can no longer perform its function that means the temperature of the pressure medium may attain excessively high values which in turn may cause damage in the whole hydraulic circuit.